Arc detection circuit

ABSTRACT

An arc detector comprising a voltage generator for detecting a voltage and an integrator for integrating said voltage with respect to time and generating an output signal corresponding to said integration. The arc detector also includes a discharge controller for controlling said output signal of said integrator, an amplifier for amplifying said output signal from said integrator and outputting an amplified signal; and a comparator for comparing said amplified output signal to a reference voltage and generating a detection signal based on said comparison.

BACKGROUND OF THE INVENTION

The present invention relates to the field of electrical devices fordetecting fault conditions, and more particularly to devices that sensethe presence of electrical arcs in a circuit. Electrical arcing is alocalized, high-energy event caused by wire chafing, dust build up andmoisture that may result in a catastrophic fire. However, arcs, likemany transients, are very short lived and therefore cause little impacton bi-metallic elements or the electronics. Consequently, conventionalcircuit breakers and electronic wire protection methods do not interruptcircuits fast enough to prevent damage or a fire. There are two distincttypes of arcs, series arcs and parallel arcs.

In general, a parallel arc starts when current flows between twotouching oppositely charged conductors causing metal at the contactpoint to liquefy and current to jump over the resulting gap. The arc maythen dissipate, but has the potential to re-establish itself at a latertime. Parallel arcs may also occur intermittently, usually as a resultof vibration, without ever turning into a hard short. FIG. 9 shows atypical arc in a 28VDC system where the arcing event occurs over severalseconds.

The arc current in a parallel arc does not pass through the load, andonly the source current capability and wiring resistance limit the peakarc current. There are several scenarios in which parallel arcs occur.For example, parallel arcs can occur when wire insulation is missing ordamaged and a wire connected to an aircraft power bus lightly contactsthe airframe ground (or another exposed wire), or when insulationlocated in a wet or dirty environment is missing or damaged.

The other type of arc is a series arc. A series arc occurs when a gap,or break in the circuit, develops in series with a load that is normallyconnected to an aircraft power bus and the load current jumps over thegap. Unfortunately, some loads utilize series arcs as part of normaloperation, so they cannot be reliably detected. For example, the normalsparking of a motor commutator cannot be distinguished from a seriesarc. Arc lamps, i.e., strobes and fluorescent lighting are also hard todistinguish from arcs.

Various types of arc detectors have been developed and/or proposed todetect arc currents. Generally, there are two types of detectors. Onetype responds to the random high frequency noise content of the currentwaveform generated by an arc. This high frequency noise tends to beattenuated, especially by the presence of filters on some loads, whichcan be connected to the branch circuit. The other basic type of arcdetector responds to the step increase in current occurring as the arcis repetitively and randomly struck. Examples of arc detectors of thelatter type are disclosed in U.S. Pat. Nos. 5,224,006 and 5,691,869.

These prior art devices have difficulty distinguishing arc currents fromnormal inrush currents. Inrush currents, e.g., capacitive inrushcurrents, lamp inrush currents, or motor inrush currents, are differentfrom arc currents in one very important respect; inrush currents aremuch more regular and predictable. Exemplary types of inrush currentsare described below.

The capacitive inrush current is exponential in shape, shown in FIG. 10,with a time constant determined by circuit resistance and loadcapacitance, e.g., an electronic device that has a capacitor bank on theinput, such as a flight computer, data acquisition box (“black box”) orsimilar piece of avionics equipment. Lamp inrush currents occur whenthere is a change in resistance from a cold filament to a hot filamentin an incandescent lamp. An exemplary waveform for a lamp inrush currentis shown in FIG. 11. However, lamp inrush currents vary with lamp type.A motor inrush current occurs when a motor starts.

An effective arc current detector will ignore all of these types ofinrush currents and only trip when an actual arc is detected. Therefore,there is a need for an improved arc detection circuit.

SUMMARY OF THE INVENTION

The instant invention relates generally to a circuit and moreparticularly to an arc detection circuit comprising voltage generatorfor detecting a voltage, an integrator for integrating said voltage withrespect to time, a discharge controller for controlling the output ofcurrent from the integrator, an amplifier for amplifying the currentoutput from the integrator, and a comparator for comparing a signaloutput from the amplifier to a reference voltage an generating adetection signal based on the comparison.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the general operation of a circuitconsistent with the arc detector of the instant invention;

FIG. 2 is a schematic illustrating an exemplary arc detection circuitconsistent with the present invention;

FIG. 3 is a graph showing a typical time versus current curveillustrating the normal peak current value;

FIG. 4 a and FIG. 4 b are graphs illustrating the operation of a circuitbreaker versus the arc detection circuit of the instant invention,respectively;

FIG. 5 a and FIG. 5 b are graphs illustrating the operation of the arcdetection circuit of the instant invention when the circuit receives acapacitance inrush current and the operation of the arc detectioncircuit of the instant invention when in the arc detection circuitreceives an arc current, respectively;

FIGS. 6 a and 6 b illustrate the operation of a circuit breaker and thearc detection circuit of the instant invention, respectively, when bothcircuits relieve a dead short from a power source;

FIGS. 7 a and 7 b are a graph and apparatus, respectively, illustratingan arc producing device known as the guillotine generating a classic DCarc; and

FIGS. 8 a and 8 b are a rotary spark gap device and a rotary spark gapcurrent waveform, respectively;

FIG. 9 is a graph showing the current detected through a load during anarcing event;

FIG. 10 is a graph showing the current characteristics of a capacitanceinrush current; and

FIG. 11 is a graph showing the current characteristics of a lamp inrushcurrent.

DETAILED DESCRIPTION OF THE EMBODIMENT

An arc detector consistent with the present invention comprises acircuit module connected across a load.

The operation of the circuit module is described with reference toFIG. 1. In voltage generator 300, a voltage is generated proportional toa voltage across the load 200. A voltage generated by voltage generator300 is output to integrator 302. The integrator 302 integrates thevoltage output from voltage generator 300, i.e., generates a signalconsistent with total voltage output from the voltage generator 300 overa given time interval. Discharge controller 304 controls current flowthrough the integrator 302, thereby ensuring current flows through theintegrator in only one direction. Amplifier 306 amplifies the signaloutput from the integrator 302, and outputs this signal to comparator308. Comparator 308 compares the amplified signal to that of a referencevoltage, and outputs a signal indicating the presence of an arc.

An exemplary circuit consistent with the block diagram of the arcdetector of the instant invention will be described with reference toFIG. 1 and FIG. 2. In the exemplary circuit, voltage generator 300consists of a current sensing resistor 1. Current sensing resistor 1 hasa voltage across it proportional to the current flowing through currentsensing resistor 1. The scaling of current sensing resistor 1, i.e., itsresistance, is chosen according so that a known voltage can be producedat a certain current. Current sensing resistor 1 may also be variable sothat large quantities of circuits may be manufactured withoutredesigning components.

Current sensing resistor 1 is connected to a voltage source 300 andintegrator 302. Integrator 302 consists of resistor 2, diode 3 andcapacitor 5. The integrator operates as follows: A current throughcurrent sense resistor 1 causes a voltage to appear across current senseresistor 1, thereby causing capacitor 5 to charge as current passesthrough resistor 2 and diode 3. When enough charge builds up in thecapacitor 5, the capacitor discharges. The integration time constant ofthe integrator, i.e. the charge time of the capacitor 5, effects themaximum allowable inrush current. In other words, the time constant isset so the capacitor 5 is not discharged when the aforementioned inrushcurrent flows through current sensing resistor 1 to capacitor 5. Theintegration time constant can be adjusted by changing the resistance ofresistor 2 and the capacitance of capacitor 5. Thus, the arc detectioncircuit is adaptable to many different types of applications.

The discharge controller 304 consists of resistor 4 operating inconjunction with diode 3. It is noted that diode 3 is used in theintegrator 302 and the discharge controller 304, though other circuitarrangements could be contemplated that would employ separate elementsin the integrator 302 and the discharge controller 304. Resistor 4 anddiode 3 control the discharge of capacitor 5. Without diode 3, capacitor5 would discharge back through resistor 2 at the same rate it charged.Diode 3 prevents the discharge through resistor 2, thereby allowingresistor 4 to discharge capacitor 5 at a slower rate. Thus, recurring,short duration, high current pulses and long duration, higher currentpulses are effectively integrated.

When capacitor 5 discharges, a current is input into amplifier 306 andamplified. The amplified signal is then output to comparator 308.Comparator 308 receives a reference voltage 7, typically 2.5 volts, andoutputs an arc signal when an arc is detected.

The normal peak current of an arc is variable, but an exemplary timeversus current curve used for determining the normal peak current of anarc and the reference voltage value (so that the circuit can be designedwith the proper resistance and capacitance values) when designing thearc detection circuit consistent with the instant invention is shown inFIG. 3.

Arc Circuit Testing

Arc currents were generated in several ways for testing the arc detectorcircuit of the instant invention. One method developed was a guillotine,shown in FIG. 7 b. The guillotine 500 was formed by using a weightedblade 502 that is hoisted to a fixed height and then allowed to droponto a zip cord 508 carrying power and ground conductors. The bladedrops onto the zip cord at the same angle and at the same velocity everytime. It slices into and shorts together the 28 volt DC and ground for ashort period of time before the springs 504 return the blade 502 to thenon-shorting position. The duration of the arcing event is determined bythe height of the blade 502's drop point, the wire insulation material,blade sharpness, and by the stiffness of the springs 504. The arcproduced by the guillotine is a classic DC arc, and is different from ahard short only by the fact that it has a non-zero and dynamicimpedance. The key to the repeatable nature of the guillotine test is inthe fact that it is not hand operated.

Another arc producing device developed was the Rotary Spark Gap device600 described in FIG. 8 a. The rotary spark gap device 600 creates amomentary arc across an operating load by brushing a wire 602 connectedto a 28 volt DC load terminal 604 against the sides of a hold 606 in aground metal contact 608. The rotary spark gap device 600 accuratelymodels a chaffed wire vibrating against bare aircraft metal. A typicaloutput waveform from this apparatus is shown in FIG. 7 b. The rotaryspark gap device simulates arcs that are caused by wires with faultyinsulation contacting the bare aircraft metal.

Using these exemplary devices, the response of the arc detection circuitof the instant invention to various transient events was determined.

The operation of the arc detector of the instant invention versus theoperation of the circuit breaker is shown in FIG. 4 a and FIG. 4 b. InFIG. 4 a, a circuit breaker receiving an arc does not interrupt an arcevent, even after 55 milliseconds. However, in FIG. 4 b, arc detector ofthe instant invention detects and interrupts the current within fourmilliseconds.

FIG. 5 a and FIG. 5 b describe the operation of the arc circuit of theinstant invention when receiving an inrush current versus when receivingan arc current. It is noted that the arc detector of the instantinvention ignores the inrush current because, by setting the capacitanceof capacitor 5 and the resistance of resistor 2, the arc detector has amaximum inrush current, e.g., 45 amps. It is noted that the arc detectordetects a longer duration arc event and trips after only a 4 or 5millisecond delay period.

The arc detector of the Applicant's invention can also be used tointerrupt a dead short from a power source. In FIG. 6 a, the currentcharacteristics of a circuit employing a conventional circuit breaker isdepicted. It is noted that the circuit breaker circuit takes over 200milliseconds to break the current. In FIG. 6 b, the arc detector circuitof the instant invention is depicted. The arc detector circuit breaksthe current after only 200 microseconds, i.e., 3 orders of magnitudeimprovement over the circuit breaker circuit.

One skilled in the art will recognize that various changes could be madeto the foregoing without departing from the spirit and scope of theinvention.

1. An arc detector comprising: a voltage generator for detecting avoltage; an integrator for integrating said voltage with respect to timeand generating an output signal corresponding to said integration; adischarge controller for controlling said output signal of saidintegrator; an amplifier for amplifying said output signal from saidintegrator and outputting an amplified signal; and a comparator forcomparing said amplified output signal to a reference voltage andgenerating a detection signal based on said comparison.
 2. The arcdetector described in claim 1 wherein said voltage generator includes acurrent sensing resistor.
 3. The arc detector of claim 2 wherein saidintegrator comprises a second resistor, a diode and a capacitor.
 4. Thearc detector of claim 3 wherein said discharge controller comprises athird resistor and said diode.
 5. The arc detector of claim 1 whereinsaid integrator comprises a resistor, a capacitor, and a diode.
 6. Thearc detector of claim 5 wherein said discharge controller comprises saiddiode and a second resistor.
 7. A method for detecting an arc comprisingthe steps of: generating a voltage; integrating said voltage withrespect to time and generating a first signal; amplifying said firstsignal and generating a second signal; and comparing said second signalto a reference voltage to generate a arc detection signal.